Your character finds an old letter.
Where do they find it? Who has written this letter? How old is this letter? Is it addressed to them?
Does this letter tell the character anything about their own personality?
Does the character show recognition of their own feelings by reading the letter?
Reveal backstory using the letter.
Do we find out about your protagonist’s character or will this advance plot?
The task is to write fragments of this letter interspersing with the character’s thoughts (if first person) or the omniscient narrator’s voice reflecting on the contents.
Feel free to stop the letter at a point of suspense.